1. Field of the Invention
The present invention relates to an ignition timing control system for an internal combustion engine, which controls ignition timing of the engine according to the in-cylinder pressure.
2. Description of the Related Art
Conventionally, an ignition timing control system for an internal combustion engine, which controls the ignition timing of the engine using the in-cylinder pressure thereof has been proposed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2003-262177. This ignition timing control system calculates the difference between the maximum value of the in-cylinder pressure detected by the in-cylinder pressure sensor and the value of the same at the top dead center position of the compression stroke, and when the difference is smaller than a predetermined threshold value, the ignition timing is determined to be too retarded, so that the ignition timing is controlled to be advanced by a predetermined value. In the other cases, the ignition timing is held at the value used at the time.
In general, the internal combustion engine sometimes experiences variation in combustion state between the cylinders thereof (i.e. variation in heat energy generated by combustion), and in such a case, generated torque varies between the cylinders, which causes variation in the engine speed and vibrations, resulting in degraded drivability. Further, when the engine is used as a power source of a vehicle, there is a fear of occurrence of surging. Variation in combustion state between the cylinders is more likely to occur when variable valve lift mechanisms are provided for respective cylinders, for continuously changing the respective lifts of intake valves associated therewith, due to differences in the play and operating characteristics of the variable valve lift mechanism, which vary from one cylinder to another.
In contrast, the ignition timing control system proposed by Japanese Laid-Open Patent Publication (Kokai) No. 2003-262177 controls the ignition timing without considering the above-mentioned variation in combustion state between the cylinders, and hence can give rise to degradation of drivability and occurrence of surging. This problem is more likely to become conspicuous particularly when the system is applied to internal combustion engines equipped with the variable valve lift mechanism.